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Excitonics at the Space-Time Limit

Gemeinsames Kolloquium des Fachbereichs Physik und des SFB 1083

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Abstract:
In 2D semiconducting quantum materials, organic semiconductors and their heterostructures, the energy of absorbed light is stored in Coulomb-bound electron-hole pairs, which are called excitons. For future technological applications of these classes of materials, for instance in optoelectronics and for energy harvesting, it is crucial to study the initial exciton formation and also the subsequent relaxation and dissipation processes at the fundamental level and on the relevant length and time scales.
In our research, we have built a new photoemission-based experiment [1] that is capable of studying excitonics at the space-time limit corresponding to nanometers and femtoseconds. In a series of experiments, we identified characteristic signatures in the exciton formation process and the pathways of subsequent energy conversion and thermalization. In addition, the new experiment gives us access to the so-called “dark exciton landscape”, where we can follow exciton dynamics with unprecedented temporal and spatial resolution.  
In my talk, I will present the ultrafast formation dynamics of dark interlayer excitons in twisted WSe2/MoS2 heterostructures. In particular, I will report on the identification of a key signature of the moiré superlattice that is imprinted on the momentum-resolved interlayer exciton photoemission signal [2]. Furthermore, I will present photoemission exciton tomography [3] that allows us to disentangle multiorbital contributions in the exciton formation of the organic semiconductor buckminsterfullerene C_60.

Schematic of twisted layers of WSe2 (top) and MoS2 (bottom). Following optical excitation, a multitude of optically “dark” excitons are created. These “dark” excitons are electron-hole pairs bound by Coulomb interaction (light and dark spheres connected by field lines), which cannot be directly observed using visible light. One of the most

interesting quasiparticles is the "moiré interlayer exciton" – shown in the middle of the image - in which the hole is located in one layer and the electron in the other. The formation of these excitons on the femtosecond time scale and the influence of the Moiré potential are investigated using femtosecond photoemission momentum microscopy [2].

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References
[1] Keunecke et al., Rev. Sci. Ins. 91, 063905 (2020).
[2] Schmitt et al., Nature 608, 499 (2022).
[3] Bennecke et al., arXiv:2305.18908 (2023).

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